I'm not sure if many folks overclock their PCIe slots (other than the fairly traditional 101 vice 100 just to ensure that the slot is freq. locked) anymore. I did some trial and error stuff a while back using info I got from Ntune (this is a long while back... haven't used Ntune in a couple of years at least) and found an actual FPS benefit in 3DMark testing to pushing the speed up.

Most overclocking guides I've seen though suggest that overclocking the PCIe slot by much will hamper CPU overclocking attempts.